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ReAENERGY UPGRADE

WHITE PAPER ON REA ENERGY UPGRADE

July, 2016

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Edited by: Alexandra Gade and Hiro Iwasaki

Layout by: Erin O’Donnell

Cover art by: Erin O’Donnell and Don Lawton

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Table of Contents 1. Introduction and Executive Summary............................................................................................................... 1

2. Evolution of structural phenomena ................................................................................................................... 7

2.1 Shell evolution in exotic nuclei probed by direct reactions .................................................................... 7

2.2 Direct reaction studies around 100 < A < 200 ........................................................................................ 8

2.3 Proton resonance scattering and direct reaction studies near the continuum .......................................... 9

2.4 Properties of weakly-bound neutron-rich nuclei probed by hadronic scattering .................................. 11

3. Correlations, collectivity and configuration coexistence in exotic nuclei....................................................... 12

3.1 Precision lifetime measurements of low-lying states with Doppler-shift techniques ........................... 12

3.2 Nuclear structure studies at medium and high spins ............................................................................. 13

3.3 Studies of neutron-proton pairing correlations in N=Z nuclei .............................................................. 15

3.4 Shape evolution in neutron-rich nuclei near the r-process path ........................................................... 16

4. Heavy elements and the equation of state ....................................................................................................... 17

4.1 The path to heavy elements at FRIB..................................................................................................... 17

4.2 Understanding the fusion-evaporation process for heavy element studies ........................................... 19

4.3 Isospin transport, nuclear dynamics, and nuclear thermodynamics ..................................................... 19

5. Nuclear Astrophysics ..................................................................................................................................... 22

5.1 Rapid proton capture process (rp process) ........................................................................................... 22

5.2 Rapid neutron capture nucleosynthesis (r process) .............................................................................. 24

6. Fundamental Symmetries................................................................................................................................ 26

6.1 Exploring octupole-deformed nuclei .................................................................................................... 26

7. Applications .................................................................................................................................................... 27

7.1 Surrogate reactions ............................................................................................................................... 27

7.2 Nuclear structure from compound-nucleus reactions ........................................................................... 28

7.3 Nuclear structure inputs for reaction rate calculations above A=200 ................................................... 29

8. Equipment ....................................................................................................................................................... 30

8.1 GRETINA/GRETA .............................................................................................................................. 30

8.2 Solenoidal spectrometer ....................................................................................................................... 31

8.3 ISLA ..................................................................................................................................................... 32

8.4 Auxiliary detection systems .................................................................................................................. 33

9. ReA energy upgrade ....................................................................................................................................... 34

9.1 Conceptual layout of the ReA6-12 experimental area .......................................................................... 34

9.2 Cost estimates for the ReA energy upgrade ......................................................................................... 35

9.3 Staging options ..................................................................................................................................... 35

ReA3 upgrade workshop .................................................................................................................................... 37

Reference ............................................................................................................................................................ 38

Contributors ........................................................................................................................................................ 41

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1. Introduction and Executive Summary

The Facility for Rare Isotope Beams (FRIB) will provide beams of rare isotopes with

unprecedented intensities and will allow scientists to map the nuclear landscape, to

understand the forces that bind nucleons into nuclei, to answer questions about the

astrophysical origin of nuclear matter and to address societal needs related to nuclear

science and technology. Central to the FRIB concept is the in-flight separation of rare

isotopes and use of these beams at low and high energies [LRP15,NSAC07]. The full

complement of the fast, stopped, and reaccelerated beam capability offers a broad spectrum

of rare-isotope science programs with beam energies from a few keV/nucleon up to at least

200 MeV/nucleon, maximizing the science discovery potential at FRIB.

The Low-Energy Nuclear Physics community has strongly endorsed the early

implementation of reaccelerated beams with energies up to 12 MeV/nucleon at FRIB

[FRIB09,NSCL09,ReA12] and as soon as possible at NSCL [LRP15,LECM15]. The

combination of in-flight separation, gas stopping and reacceleration is a world-unique

approach to produce low-energy beams of rare isotopes at sufficient energies to exploit

well-established reaction techniques and to further develop experimental and theoretical

tools necessary for emerging nuclear physics programs at NSCL and FRIB.

The National Research Council Committee posed four overarching questions for nuclear

physics [NRC13];

How did visible matter come into being and how does it evolve?

How does subatomic matter organize itself and what phenomena

emerge?

Are the fundamental interactions that are basic to the structure of

matter fully understood?

How can the knowledge and technological progress provided by

nuclear physics best be used to benefit society?

The 2015 Long Range Plan [LRP15] reaffirmed these questions and stated “since the last

Long Range Plan in 2007, we have considerably increased our understanding of the

nucleus and its role in the universe. But answers to the overarching questions that drive

the field require still deeper understanding of atomic nuclei, both theoretically and

experimentally. The breadth of the research questions requires complementary approaches

with a variety of tools and techniques”, identifying needs for a diverse set of nuclear science

programs to answer the above overarching questions.

This whitepaper presents scientific opportunities afforded by an energy upgrade to the

present ReA facility at NSCL. ReA3 is operational [ReA3], delivering reaccelerated beams

of rare isotopes to experiment as part of NSCL’s user program. Unique physics programs

have started with major equipment such as ANASEN [Lin12], JENSA [Chi14,Bar15] and

the AT−TPC [Suz12]. An energy upgrade of ReA up to 6 MeV/nucleon and eventually up

to 12 MeV/nucleon opens up a variety of new physics opportunities as outlined in this

whitepaper. The capability to produce world-unique beams of rare isotopes up to 12

MeV/nucleon is achieved at ReA through a multi-stage operation of the facility including

in-flight separation after fragmentation or fission, gas stropping, charge-breeding and

subsequent reacceleration. The use of stopping in a helium gas and charge breeding

provides a nearly chemistry-independent source of ions, thereby complementing the ISOL

technique. Low-energy rare-isotope beams with well-defined beam energies and small

emittance have been pursued at ISOL facilities worldwide, whereas no other facility than

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ReA will have the full range of elements available from in-flight separation. At NSCL, and

in the future at FRIB, state-of-the-art equipment such as GRETINA/GRETA [Pas13], a

solenoidal spectrometer [Wuo07,Lig10], the AT−TPC [Suz12], and ISLA [ISLA15] as

well as a variety of complementary detection systems will be available or are envisioned

for science programs with higher-energy reaccelerated beams.

The NRC’s Rare Isotope Science Assessment Committee (RISAC) report from 2007

[RISAC07] articulated the science drivers for FRIB encompassing a broad range of nuclear

science: nuclear structure physics, nuclear astrophysics, fundamental symmetries, and

societal applications. They are summarized below and listed together with the relevant

section numbers of this whitepaper. The physics programs enabled by a ReA energy

upgrade cover a large fraction of science drivers of FRIB.

Nuclear Structure Physics

Testing Nuclear Structure Concepts (2.4)

Probing the modification of shell structure (2.1, 2.2, 2.3)

Pairing and superfluidity (3.3)

The evolution of collective motion in complex nuclei (3.1, 3.2, 3.4)

Production and properties of the heaviest nuclei (4.1, 4.2)

Probing neutron skins (2.4)

Nuclear Astrophysics

The origin of the heaviest elements (5.2)

Explosive nucleosynthesis (5.1)

Composition of neutron stars (4.3)

Fundamental Symmetries

Test of fundamental symmetries with rare isotopes (6.1)

Other Scientific Applications

Applications for the benefit of stockpile stewardship, materials science,

medical research, and nuclear reactors (7.1, 7.2, 7.3)

To meet the scientific goals at FRIB listed above, key nuclear reactions are identified in

the RISAC report [RISAC07] that include single- and two-nucleon transfer reactions at

10−20 MeV/nucleon, single-step or multiple Coulomb excitation of heavy nuclei, fusion

and multi-neutron transfer, and (quasi) elastic scattering. These reactions are best or

exclusively studied with low-emittance beams of precisely controlled energies, which is

the exactly the scope of the energy upgrade of the ReA facility. High-quality data from

these reaction studies spanning a wide beam-energy range will also provide important

benchmarks for advanced ab-initio theory that has taken first steps to unify the fields of

nuclear structure and reactions [LRP15], enabling insights into the nature of the nuclear

force and nuclear dynamics.

FRIB will provide rare-isotope beams with unmatched intensities, exceeding what is

available today by orders of magnitude. It will make the unique reacceleration scheme for

in-flight separated beams a vital tool for providing exotic beams at the desired low energies

for experiments. Physics programs with the reaccelerated beams will complement the fast-

beam programs, in particular, aiming at aspects of the structure and reactions of exotic

nuclei only accessible in experiments at the ReA beam energies. The reach and extent of

the scientific motivations described in this whitepaper is illustrated in the nuclear chart

shown in Fig.1.1. The whole program covers the full mass range from light to medium-

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mass and to very heavy nuclei, approaching the neutron and proton driplines as well as

actinide isotopes. At the same time, a wide variety of subjects are addressed including

nuclear structure and reaction studies, nuclear astrophysics, fundamental symmetries, and

applications, all of which are essential to answer the overarching questions [NRC13]

reaffirmed by the 2015 Long Range Plan [LPR15] and for meeting the science goals at

FRIB as articulated by the RISAC report [RISAC07].

The ReA energy upgrade will provide unique beams to:

facilitate reaction studies with well-established probes, mapping out the

evolution of structural phenomena throughout the nuclear chart

reach medium- and high-spin states in neutron-rich exotic nuclei, elucidating

the interplay between neutron excess and angular momentum

extend the new-isotope discovery potential at FRIB to heavier neutron-

rich nuclei

address broad topics of astrophysics and societal applications of nuclear science

Fig.1.1: Science opportunities at ReA6−12 described in this whitepaper. Example physics cases in selected regions of interest are shown for single-particle structure (dark blue), nuclear

collectivity and configuration mixing (red), nuclear reaction studies (light blue), nuclear

astrophysics (green), and fundamental symmetries and applications (grey).

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The ReA facility provides reaccelerated beams of rare isotopes at the well-controlled beam

energies that are required for a broad set of low-energy nuclear reactions at and above the

Coulomb barrier. Compared to fast beams that are produced from fragmentation or fission

reactions and separated in-flight, the small emittance of the reaccelerated beams following

gas stopping and charge breeding is a great advantage since the required resolutions can be

achieved without beam tracking. Figure 1.2 summarizes suitable beam energies for

representative nuclear reactions and experimental techniques discussed in this whitepaper.

Experimental tools for nuclear structure studies include fusion and Coulomb excitation of

heavy nuclei at and below the Coulomb barrier and multiple Coulomb excitation at barrier

energies that require ReA6 beam energies. Direct transfer reactions such as (d,p) and (p,d)

become reliable at and above 9 MeV/nucleon, and higher beam energies are preferable to

have better momentum matching conditions for high-l single-particle states in medium-

mass and heavy nuclei. Finally, an energy of 12 MeV/nucleon or above is required for

some one- and most two-nucleon transfer reactions as well as for fragmentation-type

reactions for studies of the nuclear equation of state.

Fig.1.2: Nuclear reactions and experimental techniques envisioned for science programs

at ReA. The reaction Q values depend largely on the case of interest, therefore, the energy

information was chosen based on examples given in this whitepaper.

A pre-conceptual design of the ReA12 experimental area is shown in Fig.1.3. Three

additional cryomodules (=0.085) are planned as an extension to the current ReA3 beam

line, and each module adds about 3 MeV/nucleon beam energy. In this preliminary concept,

the experimental area accommodates two dedicated beam lines for a solenoidal

spectrometer and the ISLA recoil separator and includes one general-purpose beam line.

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A wide variety of nuclear structure and reaction studies with state-of-the-art

instrumentation are anticipated at ISLA as outlined in the ISLA whitepaper [ISLA15].

GRETINA/GRETA can be installed at the target position of ISLA or at the general-purpose

beam line together with auxiliary detectors. For reaction studies, the AT−TPC [Suz12] and

a silicon detection system as presently used in HELIOS [Wuo07] are both considered to

share the solenoidal magnet for their experiments. The general-purpose beam line serves

as the main beam line for a wide variety of complementary detection systems envisioned

by the community.

Fig.1.3: Pre-conceptual design of the ReA12 experimental area, which includes two beam

lines dedicated to a solenoidal spectrometer and ISLA as well as one general-purpose

beam line.

The expected available energies for the ReA upgrade are given in Fig.1.4. The available

maximum energy depends on the Q/A ratio of the beams and ReAX (X=3, 6, 9, 12) is

designed to provide at least X MeV/nucleon for very neutron-rich isotopes with Q/A of

0.25. This specification results in much higher beam energies for proton-rich or N=Z nuclei.

For instance, the maximum energies available for 30S at ReA12 would exceed 20

MeV/nucleon. The additional energy increase available for proton-rich beams is important

to employ transfer reactions with highly negative Q values such as (p,d) and (p,t). The

detailed maximum beam energy information for each isotope is given in Fig.1.5 for ReA6

(left) and ReA12 (right).

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This whitepaper was compiled on the basis of presentations and discussions initiated by a

one-day workshop on the science opportunities afforded by an energy upgrade to ReA3

held in August 2015 at Michigan State University. In the following, the science motivations

for a ReA energy upgrade are discussed, with opportunities encompassing nuclear structure

and reaction studies, nuclear astrophysics, fundamental symmetries, and applications. The

last section briefly summarizes the conceptual layout of the ReA12 experimental area and

provides cost estimates and possible staging options for the ReA energy upgrade.

0

5

10

15

20

25

0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55

MeV

/nu

cle

on

Q/A

85Ge

58Cr

30S

65Ge

76Ga

ReA 3

ReA 6

ReA 9

ReA 12Reaccelerator Specifications

Fig.1.4: Maximum energies expected for ReA3-ReA12 shown as a function of the Q/A ratio

of rare-isotope beams.

Fig.1.5: Maximum beam energies expected from ReA6 (left) and ReA12 (right) shown

across the nuclear chart. The dashed lines indicate the traditional magic numbers 8, 20,

28, 50, 82, and 126 to guide the eye.

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2. Evolution of structural phenomena 2.1 Shell evolution in exotic nuclei probed by direct reactions The advent of rare-isotope beams has opened the door to detailed studies of nuclei very far

from stability. It is now well established that the properties of such nuclei are very different

from those found near the valley of stability, representing the need to redefine the

paradigms of nuclear structure physics. One of the most notable observations is that the

standard shell model of the nucleus requires dramatic revisions in some regions of the

nuclear chart. While measured masses, separation energies, and electromagnetic transitions

between low-lying excited states provided the first indications of these changes, data from

nucleon-transfer reactions performed in inverse kinematics with more intense rare-isotope

beams can characterize shell structure on the microscopic level. Single-nucleon stripping

reactions such as (d,p) can be used to identify energy levels, determine their spins and

parities, deduce spectroscopic strength, and provide information about residual interactions.

Two-nucleon transfer reactions such as (3He,p), (,d), (d,) and (t,p) provide information

about pairing correlations, configuration mixing, and shape coexistence. Nucleon-removal

reactions such as (d,t) or (d,3He) are complementary to knockout reactions and able to

provide information on the occupancies of shell-model orbitals.

The most useful information for such reactions is obtained from measurements done at

energies near or above the Coulomb barrier. The appropriate energies depend on

momentum and angular-momentum matching conditions, which in turn depend on the

reaction Q value. For example, (d,p) or (t,p) reactions typically have Q values that are near

0 MeV, and require only modest beam energies between 5−10 MeV/nucleon. The most

suitable energies for deuteron transfer reactions are between 10−15 MeV/nucleon. Due to

very negative Q values, which can be less than −15 MeV, (d,3He) reactions must often be

performed at energies of at least 15−20 MeV/nucleon or higher. Only high-quality

reaccelerated rare-isotope beams with well-controlled energies and emittances have the

necessary properties for such studies.

Two important areas in the nuclear chart where the shell properties markedly deviate from

the common wisdom are very neutron-rich nuclei near N=20 (32Mg) and N=32 (52Ca). Near 32Mg – in the famous “Island of Inversion” – the shell closure at N=20 disappears.

Particularly in 32Mg, phenomena such as deformation and shape coexistence become

important. The single-particle strengths may become fragmented and the distribution of

that strength can be studied experimentally with reactions such as 32Mg(d,p)33Mg or 28Ne(d,p)29Ne. Shape coexistence and configuration as well as the impact of neutron excess

on pairing correlations in neutron-rich nuclei [Tan08] can also be studied with the (t,p) or

(p,t) reaction in inverse kinematics. Recently, the 30Mg(t,p)32Mg reaction has been studied

at low beam energy (1.8 MeV/nucleon) at REX-ISOLDE [Wim10], however, the

measurement suffered from limited resolution; hence, the conclusions are uncertain. New

measurements for this system performed at higher energies with favorable kinematic

conditions should clarify the situation.

Another region full of surprises is near 52Ca, where changes in the 1f7/2, 2p3/2 and 2p1/2

neutron shells produce alleged shell closures at N=32 and N=34 [Wie13,Ste13,Gar16].

Here, very few data exist and nucleon-transfer studies before FRIB will be challenging.

Once a reaccelerated 52Ca beam from FRIB becomes available, one- and two-nucleon

stripping and pickup measurements will be possible at ReA as well.

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2.2 Direct reaction studies around 100 < A < 200 Tracking the evolution of single-particle excitations across the nuclear chart is a key focus

of exotic-beam physics programs, which aims at establishing a unified theoretical picture

for stable and exotic nuclei. Heavier beams with A>100 will be available at ReA with FRIB,

allowing long chains of closed-shell isotopes and isotones to be studied at energies in the

proximity of the Coulomb barrier. Here, direct reactions provide crucial information. The

most suitable energy for transfer reaction studies is a few MeV/nucleon above the Coulomb

barrier in both the entrance and exit channels. At such energies, the reactions can be reliably

modeled as a direct and single-step process. The cross sections are favorable and forward

peaked, with the angular distributions indicative of different angular momenta transferred

in the reaction.

In heavier systems, for example those around tin and above, high-j orbitals lie close to the

Fermi surface. To probe them via a neutron- or proton-adding reaction, one typically needs

to employ probes such as (α,3He) and (α,t) instead of the commonly used (d,p) and (3He,d)

reactions. While these probes are equivalent in terms of adding a nucleon, the former

reactions, with the large Q values, result in more favorable angular momentum matching

for transfer to high-j states, and thus are more reliable in terms of determining the

spectroscopic information for such states. To track single-proton g7/2 and h11/2 excitations

from the lighter Sn isotopes to those beyond N = 82, one can exploit the Sn(α,t) reaction.

At FRIB, studies on 103-135Sb will be possible. Similarly, single-proton excitations near the

Pb isotopes as well as single-neutron excitations outside the N = 50, 82, and 126 isotones

can be studied via the (α,t) and (α,3He) reactions, respectively. In all cases, one requires

beams in the 10−15 MeV/nucleon range, due to the large reaction Q values as well as the

larger Coulomb barrier presented by the helium species in the entrance and exit channel.

Single-particle excitations in nuclei one or two nucleons away from stable doubly-magic

nuclei are well studied and have been used to derive effective shell-model interactions.

Fairly complete datasets are available for the stable doubly-magic 16O, 40,48Ca, and 208Pb

nuclei, and to a lesser extent 90Zr. However, no such studies have been possible for unstable

doubly-magic nuclei. With ReA, detailed studies around 56Ni, 68Ni, and 132Sn will be

possible. Taking 132Sn as an example, determining the location of each member of the

mulitplets in the particle-particle, particle-hole, and hole-hole systems (134Te, 134Sb, 134Sn, 130Sn, 130In, 130Cd, 132Sb, and 132In) allows one to extract effective interactions. The states

can be probed through neutron/proton adding and removing reactions on beams of 133Sb, 133Sn, 131Sn, and 131In projectiles. While in some cases charged-particle spectroscopy

should be sufficient, simultaneous or complementary -ray spectroscopy may be required

as the level density in these systems could be high. For example, the coupling of the g7/2

proton to the h11/2 neutron hole results in eight states which tend to span only a small

excitation-energy range. To clearly identify these states, a range of probes, which have

different matching conditions for low and high angular momentum transfers, are required.

The option to use these beams at energies exceeding 10 MeV/nucleon also opens up the

possibility of inelastic scattering reactions and transfer reactions to high-j states.

A challenge with these direct-reaction techniques is the charged-particle spectroscopy.

Beams of high purity and intensities greater than about 104 particles per second are required.

In general, the charged-particle spectroscopy in inverse kinematics requires zero-degree

recoil detection and beam separation as well as excellent beam properties in terms of energy

resolution, spatial extent, and emittance which will be available at ReA. Concerning targets,

the range of species is limited, p, d, t, 3He, 4He, etc., but once conditions such as purity,

number density, and stability are met, a diverse set of reactions can be studied for rare

isotopes available as reaccelerated beams.

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2.3 Proton resonance scattering and direct reaction studies near the continuum Properties of nuclear states in the proximity of the particle continuum may challenge shell-

model descriptions of atomic nuclei and will provide new insights in the understanding of

open quantum systems where the nuclear structure is impacted by coupling to the

continuum and particle-decay channels [Dob07]. Among complementary reaction

approaches available at ReA6−12 (Fig.1.2) for the study of nuclear structure far from

stability, proton resonance scattering in inverse kinematics has been demonstrated to be a

very effective approach to study shell structure at the proton and neutron driplines [Gol12].

The theory of resonance reactions is well understood and allows for a reliable

determination of quantum numbers and decay properties of unbound states. In proton-rich

nuclei, states of interest can be populated directly in resonance scattering. In such

experiments, large reaction cross sections and very good energy resolution (~20−50 keV

in center-of-mass energies) are typically achieved, presenting the strong potential of

resonance reactions as a spectroscopic tool. These reactions have been used extensively

over the last two decades and a large body of detailed spectroscopic information has been

obtained, including the discovery of new unbound isotopes (see for example

[Axe96,Gol10]). Similar studies can be carried out at ReA6−12, moving towards heavier

proton-rich nuclei. The desired beam energies for these studies are determined by the

excitation-energy range of interest for the specific case. The typical choice of beam energy

is between 5 and 10 MeV/nucleon and in general, heavier beams require higher energies.

This calls for an upgrade of ReA3 to at least ReA6 or ReA9 for heavier nuclei.

The resonance scattering approach can also be applied to study the structure of neutron-

rich nuclei by populating Isobaric Analog States (IAS). For example, the structure of 9He

has been explored by populating the T=5/2 IAS in 9Li in 8He+p resonance elastic scattering

[Rog03,Ube16]. One of the important features of this technique is that it allows both bound

and unbound states in neutron-rich nuclei to be probed via their analog states that appear

in neighboring isobars. The energies of IAS are shifted up with respect to their analogs;

hence, IAS can be populated by resonance scattering. As an example, the drip-line nucleus 19C is bound by 0.6 MeV with respect to neutron emission. However, the lowest T=7/2 IAS

in 19N should be unbound by about 1.5 MeV with respect to the proton decay and therefore

can be observed as a narrow resonance in 18C+p elastic scattering. For heavier nuclei, the

Coulomb shift naturally becomes larger due to the higher Z values, pushing the IAS of

interest to higher excitation energies. This requires higher beam energies for IAS studies

as compared to those required for resonance reaction studies on the proton-rich side.

Energies in the range of ReA9 will be best suited for the IAS studies of the most neutron-

rich nuclei with Z>20.

The experimental technique suited for resonance reaction studies is an active target

approach, where the detector gas serves as the reaction target. The energy of the beam

decreases as it traverses the target, and this allows reactions that occur at various center-

of-mass energies to be studied. This technique not only allows for elastic-scattering

measurements over wide energy and angular ranges but also enables studies of other

reactions, such as direct nucleon transfer reactions (p,d) and (p,t), simultaneously. The

structure of several exotic nuclei can be investigated in a single experiment, although the

center-of-mass energy resolution for transfer reactions is significantly worse than that for

resonance scattering due to unfavorable kinematics. Combining active targets with neutron

detection is particularly important for nuclear structure studies on the neutron-rich side

through the IAS approach. Due to favorable isospin coupling coefficients, the IAS in

neutron-rich nuclei predominantly decay by neutron emission through the isospin allowed

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channel(s) if such a decay is energetically allowed. Measuring this neutron decay channel

provides a clean signal for the IAS [Rog04]. This will require a new design for an active

target detector that facilitates the use of efficient neutron detection as well.

NSCL’s AT−TPC and its prototype are examples of active-target devices which have

already been used in several experiments with low-energy rare-isotope beams. Recent work

includes a study of the cluster structure of 10Be using resonant scattering of a 6He beam on

a 4He target [Suz13]. At ReA3, the first experiment using a reaccelerated beam of 46Ar at

4.2 MeV/nucleon was recently performed to investigate the single-particle strength in 47Ar

through measurements of its isobaric analog states in 47K. The trajectory of a proton

scattered off the 46Ar beam is shown in Fig.2.1, where the reaction vertex has been

reconstructed in the AT−TPC, defining the beam energy (4.0 MeV/nucleon) at which the

reaction occurred. However, the present energy limitations of ReA3 severely restrict the

variety of reactions that could be used with this new detector. Inverse-kinematics transfer

reactions such as (d,p), (p,d), (p,t), and (d,3He) are examples for which an energy upgrade

of ReA is highly desirable to realize the full potential of these high-luminosity reaction

tools. Unlike traditional experimental setups that use a thin inert target, the active-target

technique enables one to determine the reaction point in the target volume by

reconstructing the vertex of particle trajectories and hence deduce the beam energy at

which the reaction occurred. Angular distributions measured at different beam energies can

be summed up by taking into account the energy dependence of the diffraction patterns.

This feature is particularly well suited for studies of the most exotic isotopes at ReA for

which transfer reactions would be possible in a TPC with intensities down to a few hundred

particles per second.

Fig.2.1: Observed trajectory of a scattered proton (blue) from C4H10 gas used as an active

target. The track follows a spiral trajectory due to the applied longitudinal solenoid field

of 1.7 Tesla, combined with the slowing down of the proton due to its energy loss in the

gas. The projection of the proton track on a plane normal to the beam direction is shown

in red. The event corresponds to proton resonant scattering on 46Ar at 4.0 MeV/nucleon.

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2.4 Properties of weakly-bound neutron-rich nuclei probed by hadronic scattering Structure and excitation properties of atomic nuclei have often been interpreted based on

the concept of the atomic nucleus being a quantum liquid composed of protons and

neutrons. It is now well established that very neutron-rich nuclei often exhibit unusual

density profiles such as neutron skins and halos, where valence neutrons have spatially

extended wave functions [Rii94]. Unique density profiles of weakly-bound nuclei are

expected to modify dynamical properties at the limits of nuclear binding or neutron excess,

requiring measurements of quadrupole, octupole, and higher-order excitation modes in

very neutron-rich nuclei. These data are also crucial to the understanding of possible new

correlations and continuum effects in the proximity of the threshold [Fos16].

Inelastic scattering with hadronic probes provides unique opportunities to investigate

transition modes characteristic of neutron-rich nuclei. Unlike the electromagnetic probes,

which are sensitive predominantly to proton distributions in nuclei, hadronic inelastic

scattering can gauge both proton and neutron contributions to excitation. The magnitude

of the cross section scales with the deformation length of a given excitation. Inelastic

reactions directly probe low-lying excitation modes with moderate cross sections,

comparable to those in transfer reactions. Proton inelastic scattering in the low-energy

domain (10−50 MeV/nucleon) is known to have a dominant sensitivity to quadrupole

deformation of the neutron distribution [Ber81]. This feature has been utilized so far for

spectroscopic studies of neutron-rich nuclei up to A≈50 [Ril14] and similar studies can be

extended for heavier neutron-rich nuclei at ReA. Inelastic scattering with deuteron, an

isoscalar probe, is suited to extract the deformation lengths for the E2 and E3 transitions.

These measurements are best carried out at energies well above the Coulomb barrier at

10−15 MeV/nucleon. Key studies at ReA would be on nuclei around A=140 where

octupole correlations are strong and on nuclei in regions where shape evolution and

coexistence occur.

Elastic-scattering measurements are also important to determine parameters for optical-

model potentials which are required to interpret data from inelastic scattering and transfer

reactions. The optical-model potential is also useful to evaluate effects due to unknown

levels in the proton resonance scattering [Ube16]. Besides, analyzing powers for proton

elastic scattering can be measured using a polarized proton target, which provides a unique

way to investigate the neutron skin and its influence on optical model potentials. From the

recent measurement on 6,8He [Sak13], a possible modification of the spin-orbit potential is

suggested and ascribed to diffused density distributions in these neutron-rich nuclei. The

availability of rare-isotope beams at sufficiently high energies facilitates a diverse set of

well-established direct reaction approaches, thus providing crucial tools to characterize the

dynamics of neutron-rich nuclei and identify emerging phenomena at the limit of stability.

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3. Correlations, collectivity and configuration coexistence in exotic nuclei One of the challenges in nuclear science is to understand how the nucleus, a complex

fermionic many-body system, can be described in terms of degrees of freedom of its

constituents and, conversely, how simple patterns, like rotational and vibrational spectra,

emerge. As outlined in the following sections, these challenges can be addressed by a

multi-pronged approach based on an energy upgrade of ReA3. Precision excited-state

lifetime studies for low-lying states, medium- and high-spin spectroscopy, and explorations

of the elusive proton-neutron pairing correlations address the emergence of regularities

from complex many-body correlations. The last section points out the importance of

benchmarking the theoretical description of nuclear shapes in regions relevant to the r-

process where our understanding of astrophysical scenarios largely relies on nuclear theory

inputs.

3.1 Precision lifetime measurements of low-lying states with Doppler-shift techniques The modified shell structure will induce a competition between normal and intruder

configurations in the vicinity of nuclear ground states, which results in collective

phenomena such as shape coexistence and enhanced nuclear deformation [For13].

Precision lifetime measurements with Doppler-shift techniques are particularly important

to access low-lying excited states with picosecond or sub-picosecond lifetimes, which

provide key experimental inputs to characterize collective modes. The energy upgrade for

the reaccelerated beams at NSCL and in the future FRIB from 3 MeV/nucleon to 12

MeV/nucleon considerably increases the potential of excited-state lifetime measurements

with reaccelerated beams, either populated through Coulomb excitation, inelastic

scattering or transfer reactions.

Towards the neutron dripline, the single-particle structure of exotic nuclei is expected to

evolve drastically due to the enhanced role of the proton-neutron interaction and weak-

binding effects [Ots05,Dob07,Sor13,Hof14]. A good example of this evolution in

medium-mass nuclei can be found in the behavior of the 3s1/2 orbital across the nuclear

chart. In the vicinity of the stable Sn isotopes at N=70, the 3s1/2 strength dominates the

ground-state configurations. However, the 3s1/2 energy is expected to be lowered

significantly with respect to other higher-l orbitals as moving toward the lower-mass

neutron-rich region. Wood-Saxon models with weakly-bound potentials for neutron-rich

nuclei [Oza00,Hof14] indicate that the 3s1/2 strength could appear already in the

configurations of low-lying states in very neutron-rich nuclei at N=40-50. If this picture is

true, the 3s1/2 state will play a significant role in defining the evolution of the shell structure

in medium-mass nuclei and even have a significant impact on the location of the drip-line

of the Ca isotopes beyond N=40 [Hag13]. It may also induce exotic nuclear collectivity

and valence-neutron correlations in these Ca isotopes as characteristic features of weakly-

bound nuclei [Hag13,For13]. To draw a complete picture, it is important to accumulate

experimental information for the 3s1/2 state and track its single-particle strength from the

stable Sn region toward the dripline.

The energy upgrade at ReA will allow a diverse set of spectroscopy and reaction studies to

be performed to locate and quantify the 3s1/2 strength across the nuclear chart. A particularly

important region to identify the 3s1/2 strength is the neutron-rich medium-mass region

spanning from the stable Sn isotopes at N=70, via the N=50 region around 78Ni, and to the

exotic Ca isotopes near the dripline (as shown in Fig. 1.1). Excited-state lifetime

measurements are well suited to identify the single-particle s1/2 states through studies of the

l-forbidden M1 transitions expected to occur between the neighboring 3s1/2 and 2d3/2

orbitals. The relevant transitions are known in stable 116,118Sn isotopes and expected to

13

occur in neutron-rich Pd and Cd isotopes. These experiments can be performed by applying

the Doppler-shift attenuation method (DSAM) or recoil distance method with reaccelerated

beams at ReA6−12. Inverse-kinematics transfer reactions such as d(AZ,A+1Z)p and

p(AZ,A−1Z)d can be employed to populate the excited states of interest. An energy upgrade

towards 12 MeV/nucleon facilitates the detection of light recoiling particles or beam-like

reaction products which is essential to identify reaction channels. In addition, the

measurements at high beam velocities, where stopping powers are best characterized, can

reduce the systematic uncertainties considerably [McC09].

3.2 Nuclear structure studies at medium and high spins The experimental investigation of high-spin states in atomic nuclei has shown that simple

patterns emerge in nuclear spectra, indicative of rotations, vibrations, and transitional

behavior, posing questions as to how such simplicity can develop from the complex

interactions between individual nucleons. Furthermore, a possible interplay between

neutron excess and angular momentum in collective phenomena adds another interest,

calling for the experimental investigation of nuclear structure at medium and high spins

across the nuclear chart.

The identification of high-spin states which often cannot be accessed in -decay or

reached with direct reactions has provided key information across a broad excitation

energy range and up to high angular momentum. These types of studies have been carried

out on the proton-rich side using fusion-evaporation reactions with stable beams at beam

energies between 5−6 MeV/nucleon which can leave the residual nucleus in an initial,

excited state at a spin in excess of 40 ħ. The de-excitation of the nucleus occurs via the

emission of -rays, which can be measured using large -ray spectrometers such as

Gammasphere or now GRETINA/GRETA. One classic example is shown in Fig. 3.1 for 152Dy where excited states associated with spherical, triaxial, and superdeformed shapes

have been identified.

At ReA6, the major focus of studies will be on neutron-rich nuclei whose collective

behavior at higher-spin levels is not well understood. While fusion-evaporation reactions

excite the nucleus to the highest spins, they also heavily favor neutron over proton

evaporation, thus limiting the applicability of these reactions when probing the neutron-

rich region. However, the ReA6 upgrade will provide the capability of re-accelerating the

isotopes one wishes to study, and thus excited states can be probed by the direct excitation

of the rare-isotope beam interacting with a high-Z target (“unsafe” Coulomb excitation

above Coulomb-barrier energies). It has been shown that the best energies to maximize

the reaction cross section and the input angular momentum are at 5−6 MeV/nucleon.

Using these reactions, one has identified states at spin values as high as 38 ħ in 232Th with

cross sections on the order of 100’s of mb [Abu02]. Alternative reaction to access higher-

spin states in neutron-rich nuclei is deep-inelastic elastic scattering between neutron-rich

projectiles and heavy-target nuclei, while the clear identification of reaction products

using a recoil separator such as the planned ISLA [ISLA15] would be necessary to identify

elusive events.

14

Fig.3.1: Detailed level structure for the neutron-deficient isotope 152Dy obtained from -ray spectroscopy highlights co-existing collective and non-collective motions in atomic

nuclei. The ReA energy upgrade will provide access to medium- and high-spin states of

neutron-rich nuclei, addressing effects of extra neutrons on collective phenomena at high-

spin.

GRETINA will provide excellent Doppler reconstruction which will be necessary since the

reaction products emit rays while moving at substantial velocity. Utilizing a detector such

as CHICO2 to measure the angle of the scattered beam relative to the beam direction will

also enhance the final -ray energy resolution. In the long term, a complete GRETA with

~25% photo-peak efficiency will provide sufficient statistics to study medium- and high-

spin states even with only 1×104 particles per second on target.

While level structures as detailed as those shown in Fig. 3.1 would not be delineated at the

lowest rare-isotope beam intensities, yrast states in the 10−20 ħ region can still be identified.

Such information would allow for the study of the interplay of collective and non-collective

states in both even-even and odd-mass isotopes on the neutron-rich region. This new

opportunity thus adds both spin and isospin dimensions to our investigation of the evolving

nuclear structure, providing a comprehensive understanding of nuclear patterns manifested

in complex nuclei.

15

3.3 Studies of neutron-proton pairing correlations in N=Z nuclei On the proton-rich side of the nuclear chart, the role of neutron-proton (np) pairing

correlations in N=Z nuclei remains a subject of much interest in nuclear-structure physics

[Fra14]. Assuming charge independence of the nuclear force, isovector (T=1) np pairing

should exist on an equal footing with T=1 nn and pp pairing. However, it is still an open

question whether there exist strongly correlated isoscalar (T=0) np pairs (deuteron-like

pairs) in atomic nuclei. A number of theoretical studies have examined the possible T=0

np pairing condensation in N=Z nuclei and resultant collective effects [Mac00,Gez11],

whereas clear signals have not yet been identified in experiments.

Two-nucleon transfer reactions such as (t,p) and (p,t) provide a unique tool to probe pairing

correlations in nuclei [Bro73,Tan08], and suggest that the transfer of an np-pair from even-

even to odd-odd self-conjugate nuclei may stand out as the best tool to study these

correlations. The (p,3He) and (3He,p) reactions appear to be the best choice since the np-

pair can be transferred in both T=0 and T=1 isospin channels. Beyond 40Ca along the N=Z

line, these studies require rare-isotope beams and the use of inverse-kinematics techniques.

In particular, the feasibility of the (3He,p) reaction in inverse kinematics has been

demonstrated in a series of experiments at the ATLAS facility [Mac02] using a gas-cell

target and a simple setup with an annular segmented silicon detector. In these experiments,

one measures cross sections for np transfers from an even-even projectile to the lowest

J=0+, 1+ states in the odd-odd neighbor. While absolute cross sections will be of interest,

it is noted that the cross-section ratio of (0+)/(1+) itself is sensitive to the isospin

character of the pair correlations, thus providing reliable experimental evidence which is

free from systematic uncertainties due to an absolute normalization.

Reaccelerated N=Z beams, with energies of 5−20 MeV/nucleon, will only become

available with the ReA6−12 upgrade at FRIB and offer the unique opportunity to study the

(3He,p) and (p,3He) reactions using beams from 40Ca to 88Ru. Complementary reactions of

(,d) and (d,) can also be studied. The Active Target – Time Projection Chamber

(AT−TPC) constructed at NSCL [Suz12], combining both target and detection systems in

a single device, will be the perfect instrument to carry out such a program. A region of

Fig.3.2: Estimated yields for reaccelerated N=Z beams. Using the AT−TPC, beam

intensities of ~103 particles per second (yellow shaded area) will be required to study

possible superfluidity (collective pairing states) expected in open-shell nuclei. A

systematic study up to 88Ru will become possible. Red dashed lines indicate the N=Z closed

shells at 56Ni and 100Sn.

16

interest is illustrated in Fig.3.2, where the expected yields are shown as a function of the

mass of the beam. A successful measurement will require intensities of ~103 particles per

second, thus reaching nuclei in between the closed shells of 56Ni and 100Sn, the region where

collective pairing effects (superfluidity) are expected to fully develop. These systematic

studies of np-pair transfers are likely the only probe that could address the fascinating np

pairing mode in atomic nuclei.

3.4 Shape evolution in neutron-rich nuclei near the r-process path In regions around magic numbers, spherical shell closures and nuclear shape changes have

significant impacts on nuclear masses, thereby affecting astrophysical scenarios such as

the abundances and the origin of heavy elements in the r process [Mar16]. The strength of

the N=82 shell closure has a major impact on the nucleosynthesis of elements with mass

number A~130 since it affects the neutron capture and -decay rates for neutron-rich nuclei

along the r-process path in the neighborhood of Z=40. The N=82 shell closure can be

probed by the spectroscopy of neutron-rich nuclei near the r-process path with mass

numbers ranging from A~100 to A~120 as studied in previous works measuring the prompt

-ray emission of fission fragments. For example, the population of ground-state and K=2

bands up to spin 22+ and 19+, respectively, was observed for 112Ru (T1/2=1.75 s) by using

the 238U(,2nf) reaction with Gammasphere/CHICO [Wu06]. In addition, the lifetimes of

states with spins between 8+ and 16+ in 112Ru were measured using the spontaneous fission

of 248Cm [Smi12] and 252Cf [Sny13]. Thus, the strength of the quadrupole deformation can

be easily determined from the excited-state lifetimes. However, theoretically, nuclear

shapes are expected to evolve rapidly in the vicinity of 112Ru [Nom10] and the sign of

quadrupole deformation has not yet been obtained, since it requires the realization of

Coulomb excitation of the reaccelerated 112Ru rare-isotope beam at sub-barrier energy

(around 4.0–4.5 MeV/nucleon). The determination of the sign is essential for studying the

shape evolution in neutron-rich nuclei adjacent the r-process path, which is sensitive to the

strength of the shell closure at N=82 and the single-particle structure around.

To estimate the requirements for a successful and quantitative measurement that would

advance our knowledge on shape evolution, a numerical calculation of the Coulomb

excitation for 112Ru at 460 MeV bombarding energy on a 208Pb target of 1 mg/cm2 was

carried out with a semi-classical code GOSIA [Czo83] based on the GRETINA/CHICO2

setup [Wu16]. Assuming a beam intensity of 105 particles per second and a reaction cross

section of 1 mb, a statistical uncertainty ~10% can be achieved for the -ray yields in a 3-

day experiment. With this lower bound set, it is possible to reach a precision measurement

for the transitional matrix elements up to spin 8+ or 10+ of the ground-state band and up to

6+ of the K=2 band. Quadrupole moments up to spin 6+ of the ground-state band and up to

4+ of the K=2 band can also be determined. Therefore, the sign and magnitude of

quadrupole deformation as well as possible triaxiality would be well characterized based

on the measured electromagnetic properties. The evolution of the shape degrees of freedom

along the neutron-rich Ru isotopes with neutron number N up to 74 can be explored at the

upgraded ReA facility. This study can be extended to N=76 and beyond once FRIB is

online and the new instruments GRETA and CHICOx are available, where the latter would

be a modified version of CHICO2 to be coupled with GRETA. This unique instrument is

useful not only for the proposed Coulomb excitation work but also for any experiment

requiring the measurement of two-body kinematics, such as quasi-elastic reactions, deep-

inelastic reactions, and fission.

17

4. Heavy elements and the equation of state Low-energy reactions have provided essential information for the synthesis of super-heavy

elements as well as the understanding of the nuclear matter equation of state. However, the

role of the proton-neutron asymmetry in these reactions is still an open question, causing

large uncertainties in our understanding of the reaction mechanisms. Reaccelerated rare-

isotope beams facilitate reaction studies with a wide range of isotopes from neutron-

deficient to neutron-rich regions, providing experimental inputs to examine the proton-

neutron asymmetry dependence of nuclear-reaction observables. In this section, prospects

for future studies of fusion-evaporation reactions and multi-fragmentation at ReA are

described.

4.1 The path to heavy elements at FRIB The unmatched intensities of the fast rare-isotope beams that will be available at FRIB

provide tremendous discovery potential, enabling the production of new isotopes from in-

flight fission or fragmentation reactions. However, the rare-isotope production with fast

beams is limited to elements up to uranium, and, therefore, low-energy reactions with rare-

isotope beams such as fusion reactions and multi-neutron transfer reactions have recently

attracted attention as a possible path to the synthesis of new neutron-rich moderately-heavy

nuclei [Lov07].

The cross section for producing a heavy evaporation residue (EVR) depends on the spin-

weighted product of the capture cross section σcap(Ecm, J ), the fusion probability PCN, and

the survival probability Wsur. To understand and predict the formation of heavy nuclei, one

must know each of these three factors (σcap, PCN, Wsur) and their isospin dependence. While

the intensity of rare-isotope beams represents a central issue in the heavy-element synthesis,

enhanced fusion cross sections have been suggested for reactions with neutron-rich projectiles

due to a lowering of the fusion barrier as well as neutron flow effects [Lov07]. Excess neutrons

in reaction systems are also expected to increase the survival probability of heavy reaction

products due to the reduced fissility and the lower excitation energies of the products.

While it is understood that FRIB may not be able to synthesize new super-heavy elements, there

are excellent opportunities for using FRIB to synthesize new neutron-rich heavy nuclei. Most of

the use of FRIB in this area will involve making neutron-rich isotopes of elements 103−108

using neutron-rich light beams like 20−24O with neutron-rich actinide targets. Recently, there has

been interest in using FRIB beams in the Ar − Ca region to do these syntheses since available

intensities for reaccelerated beams can be as high as 107 particles per second. Therefore, the

possibility to test various features of heavy-element synthesis using the potassium beams made

available at ReA is especially exciting and relevant. Clearly, ReA3 allows one to study relevant

capture reactions with targets in the Ta − Pb region. Reaction studies with actinide targets require

the higher beam energies of ReA6.

Recently relevant capture reactions were studied at ReA3 using both neutron-deficient and

neutron-rich K isotopes. Preliminary results are shown in Figs.4.1, 4.2. A possible fusion

enhancement at barrier energies is observed for 46K, representing promising data for the use of

neutron-rich projectiles in the heavy-element production. At ReA6−12, if beam intensities of

104 particles per second are available, all targets (including Ta, Pb, and neutron-rich actinide

targets) can be used for such reaction studies. New data will refine our understanding of the

isospin dependence of the fusion-evaporation reaction mechanisms, paving the way for the

synthesis of new neutron-rich heavy nuclei at FRIB.

18

Fig. 4.1: The capture-fission excitation function for 39K + 181Ta [Wak16]. Model predictions

are made using the fusion calculator of Zagrebaev et al. [NRV], and the statistical model code

PACE4 [Gav80,Tar03].

Fig.4.2: The capture-fission excitation function for 46K + 181Ta [Wak16]. Model predictions

are made using the fusion calculator of Zagrebaev et al. [NRV].

19

4.2 Understanding the fusion-evaporation process for heavy element studies The upgrade from ReA3 to ReA6 provides a diverse set of the beam-target combination,

allowing for systematics studies of fusion-evaporation reactions as well as detailed

investigation of the reaction process which is modeled theoretically by the product of three

factors: the capture cross section cap, the fusion probability PCN, and the survival

probability Wsur. Although the final reaction cross sections are reproduced in calculations

by several groups, there is still substantial disagreement on the contributions of the

individual terms and the relative importance of certain phenomena. This is largely due to a

lack of experimental data and therefore dedicated experimental studies are called for.

A number of super-heavy elements have been discovered during recent decades using the

fusion-evaporation mechanism, where a medium-mass projectile completely fuses with a

heavy target to form a compound nucleus with moderate excitation energy (40–50 MeV)

followed by particle emission. The production of new elements in recent years has been

dominated by the use of 48Ca projectiles reacting with actinide targets [Oga15]. There has

been research suggesting that neutron-rich projectiles may lead to enhanced fusion cross

sections [Lov06, Lia03], but additional measurements are needed to confirm this. The

Coincident Fission Fragment Detector at NSCL is capable of making such measurements

and should continue to be utilized. This device is also capable of measuring PCN, which is

by far the most poorly understood among the three factors to describe the fusion-

evaporation process. There is an order-of-magnitude uncertainty in theoretical calculations

of PCN [Yan13], and the calculations often depend severely on projectile energies [Siw08].

Characterizing this term is therefore critical to understanding the fusion-evaporation

mechanism and hence excitation functions.

The understanding of the final term, the survival probability Wsur, can also be improved by

experiments using ReA beams. The survival probability cannot be measured directly, so it

is typically estimated using transition-state theory. Recent work has suggested that

collective effects increase the fission decay width of the compound nuclei [Wer15,May14],

but the interpretation depends on correct estimation of fission barriers. It would be

extremely helpful to estimate the fission barrier parameter Bf by studying electron capture-

delayed fission, where the decay populates excited states above the effective barrier. This

work could be complemented by direct measurements of the fission probability n/tot

(such as [Yan14]), which requires a combination of experiments with stable and neutron-

deficient beams.

A large number of neutron-deficient beams are already available at ReA3 at energies

slightly below and above the Coulomb barrier (5 MeV/nucleon) for these studies. The

upgrade to ReA6 would make more important neutron-rich beams available at all energies

of interest, contributing significantly to the understanding of all three steps of the fusion-

evaporation mechanism.

4.3 Isospin transport, nuclear dynamics, and nuclear thermodynamics The nuclear matter equation of state (EOS), which arises from the microscopic interactions

of nucleons, represents the relationship between temperature, internal energy, density and

pressure of the bulk nuclear matter. The nuclear EOS is of considerable interest due to its

importance in understanding heavy-ion collisions and astrophysical phenomena such as

properties of neutron stars. The dependence on the neutron-proton asymmetry represents

20

the largest uncertainty in our knowledge of the EOS and, therefore, research programs with

beams and devices available at ReA will focus on this aspect.

In earlier studies on intermediate-energy heavy-ion collisions, it has been observed that the

relation between the temperature and the excitation energy of nuclear system (often

referred to as the nuclear caloric curve) shifts to lower temperatures as the neutron-proton

asymmetry increases [McI14]. Moreover, the density−energy correlation is dependent on

the asymmetry, which implies the critical point also depends on the asymmetry [McI14].

An independent assessment of this asymmetry dependence of the EOS can be obtained

through ongoing studies of Kr+C fusion reactions. The fusion reaction studies at ReA will

provide an excited system with well-controlled asymmetry and excitation energy. The

measurements can also cover a wide range of the proton-neutron asymmetry in the reaction

system, providing the cleanest way to establish the isospin dependence of nuclear

temperatures and densities. By varying the beam energy, one can control the excitation

energy of the compound nucleus. For instance, a beam energy of 18 MeV/nucleon (ReA12

energies) for 74Kr and 88Kr would allow for excitation energies up to E*/A = 2.2 MeV in

the total internal energy of the composite Kr+C systems. For experiments, a charged-

particle detector array needs to be employed in combination with a large-acceptance

spectrometer (e.g. ISLA) proposed at ReA12. Additional space around the target would

allow for coincident detection of neutrons and hard rays (10<E<70 MeV) which provide

complementary probes of the thermodynamics quantities.

In addition to equilibrated systems mentioned above, nuclear systems out of equilibrium

also provide important constraints on the EOS. The evolution of these systems toward

equilibrium is sensitive to details of the nuclear potential driving the equilibration, for

instance, the neutron-proton transport can probe the gradient of the nuclear potential

[Sou89]. Recent works [Sou14, May15] studied details of N/Z equilibration in heavy-ion

collisions using stable beam and target combinations. Model calculations successfully

reproduce the observed trends, but do not show a strong sensitivity to the density

dependence of the symmetry potential. Extension of these studies to systems with extreme

neutron-proton asymmetries using rare-isotope beams is therefore an attractive step for the

EOS studies since it could place stronger constrains on the asymmetry term in the EOS.

Examining a neutron-deficient system (with N<Z) would also allow for the first direct tests

of the isospin symmetry, which is often assumed in describing the equilibrium processes.

Past studies have focused on the isospin equilibration between the target and projectile,

while the equilibration can also occur within an excited and deformed projectile-like

fragment [Hud12, Bro13]. Very recently, the first direct evidence was obtained [Jed16]

that the timescale for the equilibration within the projectile-like fragment is as long as a

zepto (10-21) second. This is shown in Fig.4.3, where the (N−Z)/A composition of the two

fragments emitted from projectile-like fragment is shown to approach each other as the

breakup angle increases. Here, a larger breakup angle indicates a longer contact time

between the fragments, therefore resulting in more equilibration. All of these observations

can be used to constrain the asymmetry term in the equation of state. The next generation

of these type of experiments can be performed at ReA12 with extremely asymmetric pairs

of heavy-ion beams like 74Kr, 88Kr, 53Co, and 64Co. The 53Co beam is particularly interesting

since it provides a case with N<Z for the first time to study the equilibrium process in

neutron-deficient systems. In experiments, a coincidence measurement for light charged

particles, heavy fragments, and neutrons is essential, that requires a combination of large-

21

scale equipment including a silicon array, the large acceptance spectrometer ISLA, and

neutron detection systems such as MoNA-LISA, LENDA or VANDLE.

Fig.4.3: Equilibration of N/Z in binary breakup of a largely deformed projectile-like

fragment [Jed16]. The composition (N−Z)/A is plotted as a function of the breakup angle

. The time scale for the equilibrium is on the order of a zeptosecond.

22

5. Nuclear Astrophysics 5.1 Rapid proton capture process (rp process) Classical novae and Type I X-ray bursts (XRBs) are explosive stellar events that occur in

binary star systems where hydrogen-rich matter is accreted onto the surface of a white

dwarf or neutron star, respectively. As the accreted matter builds up, nuclear reactions

cause temperatures to increase until breakout from stable burning occurs and

nucleosynthesis proceeds along the proton-rich side of the chart of nuclides via

thermonuclear runaway. In the case of classical ONe novae, peak temperatures of

approximately 0.1–0.4 GK are reached synthesizing nuclei up to 40Ca, while XRBs reach

higher temperatures, Tpeak=1 – 2 GK, and produce nuclei perhaps up to the SnTeSb region

at A~100 [Ili02,Sch06].

As these are the most common explosions in the Galaxy, there is a wealth of observational

data on XRBs that can be directly probed by astrophysical models that rely on accurate

nuclear physics data, including nuclear masses, -decay lifetimes, and reaction rates. This

allows direct comparisons of the predicted effects of specific reaction rates to observations

of these astrophysical events, and indeed it has been shown (Fig.5.1) that specific reactions

can have significant effects on light curves, final elemental abundances, and energy output

of both novae and XRBs [Ili02,Fis04,Par08,Amt08,Cyb16].

Classical novae and Type I X-ray bursts are driven by a series of (p,) reactions and -

decays (the rapid proton capture or rp process), the majority of which occur on unstable

isotopes making direct measurements challenging. While recoil separators, such as the

TRIUMF DRAGON system, have allowed for a handful of direct (p,) measurements (e.g.

[Rui06]), in the majority of cases sufficient intensities of the rare-isotope beams needed

are not yet available. Therefore, these reaction rates, which are often dominated by isolated,

narrow resonances, must be calculated using energies, spins, and particle partial widths

determined via transfer reactions and other methods. While the planned, state-of-the-art

recoil separator (SECAR) coupled to FRIB will be able to measure many of these (p,) reactions directly (up to A~64), indirect studies will still be necessary as they provide the

resonance energy and spin information needed to guide such direct measurement

campaigns. In addition, there will still be a need for indirect studies to provide the nuclear

data required to calculate reaction rates where direct measurements remain unattainable.

While direct reaction rate measurements of astrophysical importance typically require

relatively low-energy beams (Ebeam ≤ 3 MeV/nucleon), transfer reaction measurements

used to populate resonances of interest are performed at energies closer to or above the

Coulomb barrier. Furthermore, many of the compound nuclei of interest are far from

stability and require unstable beams for transfer reaction measurements. As such, higher-

energy rare-isotope beams from energy upgrades to ReA3 are needed.

In the case of classical novae, nucleosynthesis proceeds close enough to stability that some

experimental information exists on many of the reactions involved. However, several key

reaction rates still have large associated uncertainties such as 30P(p,)31S, which may vary

by as much as a factor of 20 over peak temperatures in novae due to the uncertainties in

spin assignments and the lack of partial width and spectroscopic information

[Par11,Wre14]. The situation in XRBs, where the rp process proceeds farther from stability,

is much more uncertain and there is very little experimental information available on the

(p,) reaction rates of interest, including those where direct measurements will remain

inaccessible into the FRIB era (e.g. of those at A > 64).

23

Fig.5.1: Changes in X-ray burst light curves induced by variations in (p,) reaction rates

[Amt08]. The reaction rates are increased or decreased by a factor of 100.

A variety of instrumentation currently available and planned for the future will allow

transfer reaction studies to be carried out on unstable beams that will be available from a

ReA energy upgrade. A recent study of the 57Cu(p,)58Zn reaction via 57Cu(d,n)58Zn with

GRETINA [Lan14] gives an example of one type of measurement that would be possible

with ReA beams in the future. With the increased availability of beams farther from

stability, the (d,n) proton transfer reaction can be used to selectively populate proton

capture resonances in rp process reactions of interest, such as 59Cu(p,)60Zn, 61Ga(p,)62Ge,

and 65As(p,)66Se. Using -ray detection with GRETINA, or GRETA in the future, in

coincidence with heavy recoil detection with, for example, the planned ISLA recoil

separator would allow determination of the excitation energies and spins of the states of

interest. Furthermore, the study of transfer reactions coupled with -ray detection can also

be used to determine –ray partial widths needed to calculate resonance strengths. In

addition, proton- and neutron-adding reactions, such as (3He,d) and (d,p), respectively, can

be used to determine the partial widths of proton capture resonances. For example, a

HELIOS-type device coupled with ReA6−12 would allow a study of resonance strengths

via the 30P(3He,d)31S reaction. Both reactions that require a gas target, such as (3He,d), as

well as those where high target purity is needed may also make use of gas jet targets, such

as JENSA [Chi14].

The availability of rare-isotope beams at energies higher than currently available at ReA3

will have a major impact on our understanding of the nuclear reactions that govern the rp

process. While direct measurements of the reactions of interest are desirable, information

on proton-capture resonances, such as resonance energies and strengths, is needed in order

to have such a program be successful. Furthermore, even in the FRIB era, certain reactions

will remain inaccessible via direct means. Thus, a strong program of indirect measurements

of astrophysically relevant reactions is crucial now and will continue to be so well into the

future.

24

5.2 Rapid neutron capture nucleosynthesis (r process) Recent precision measurements of elemental distributions of the envelopes of individual

low-metallicity halo stars are placing unprecedented constraints on heavy-element

abundance patterns [Sne08] produced through rapid neutron capture (see Fig.5.2). The

rapid neutron-capture process (r-process) occurs in high entropy environments (with core-

collapse supernovae and neutron-star mergers being the leading candidates for the sites) in

which intense neutron fluxes result in extremely rapid, successive neutron captures, driving

the populated isotopic distribution very neutron-rich. These isotopes eventually -decay

back toward stability via a network of reactions, thereby contributing to the observable

abundance pattern of stable (and very long-lived) nuclei. However, understanding r-

process abundances requires a variety of physics inputs, including neutron densities, fluxes,

temperatures and entropies of r-process sites. Additionally, considerable sensitivity has

been demonstrated to the properties of neutron-rich nuclei, such as nuclear masses, -decay

lifetimes, neutron-capture rates, and fission properties. Due to the unavailability of

neutron-capture cross sections on many of these very short-lived nuclei, constraints must

come from nuclear structure models, which need to be calibrated in the neutron-rich region.

Key to this endeavor is a systematic program of spectroscopic measurements of states with

single-neutron character, particularly in the vicinity of the N=50 and 82 shell closures.

Furthermore, network calculations of late-stage r-process nucleosynthesis indicate that the

final abundance pattern is sensitive to neutron-capture cross sections on a particular subset

of nuclei (see, for example, Fig. 5.3) at the onset of the fragmentation of single-particle

strengths around shell closures, including 81Ni, 76Cu, 78Zn, 80Ga, 86,88As, 131,133,135Cd, 133,135,137Sn, and 137Te [Sur09,Sur14]. Spectroscopic measurements on such individual

nuclei with particular r-process sensitivity are required to more directly constrain

calculations of their neutron-capture cross sections.

Single-neutron transfer reactions which selectively populate states of importance for

neutron capture, can be used to constrain neutron-capture cross sections, and are critical in

benchmarking nuclear structure models, yielding excitation energies and spin-parity

assignments, along with spectroscopic factors required for constraining both structure

models and calculations of neutron-capture cross sections. Beam energy, however, is

critical to such transfer-reaction studies, as sufficient energy is required to ensure

distinctive angular distributions and reliable reaction theory. Particularly for nuclei around

the N=82 shell closure important for the r process, such reactions require beam energies of

around 10 MeV/nucleon or higher to be performed optimally. As such, this research

program depends critically on the proposed energy upgrade to ReA being realized.

25

Fig.5.2: Abundances determined from independent high-resolution spectral measurements

of five halo stars (offset vertically for display purposes) [Cow06]. The consistency of the

data with the Solar System (SS) r-process abundances strongly suggests that a unique r-

process exists in nature.

Fig. 5.3: Sensitivity study of global abundances in an r-process event to nuclear structure

properties; the color (light to dark) shows increasing sensitivity of global abundances to

the capture rate on individual isotopes in the A=132 region [Sur09]. Single-neutron

transfer reactions at ReA provide important constraints on nuclear structure models

around the N=82 shell closure and hence on calculations of neutron-capture cross

sections.

26

6. Fundamental Symmetries 6.1 Exploring octupole-deformed nuclei Octupole-deformed pear-shaped nuclei are reflection asymmetric and have low-lying

excited states of opposite parity with respect to that of the ground state (Fig. 6.1). These

two features amplify the observable effect of parity violating interactions originating within

the nuclear medium [Hax83], which makes them attractive candidates for tests of

fundamental symmetries. As one of such examples, electric dipole moment (EDM) [Spe97]

is a particularly clean signature of new sources of CP-violation [For03], which are needed

to explain the matter-antimatter asymmetry of the universe and are predicted by various

extensions to the Standard Model such as supersymmetry.

Atomic EDMs of octupole-deformed species are expected to be orders of magnitude larger

than those of spherical nuclei such as 199Hg [Gri09], which currently sets the best limits on

CP-violating interactions within the nucleus. The sensitivity enhancement compared to 199Hg depends on the size of the quadrupole and octupole deformations and inversely on

the energy splitting of the parity doublet. EDM searches are already underway in some

octupole-deformed nuclei such as 225Ra [Par15] and 221,223Rn, but more sensitive candidates

may exist.

At FRIB, Coulomb excitation measurements on the EDM candidates will be possible with

GRETA and ReA beams. Since the charge breeding of nuclei in the A~200 region will

result in small Q/A values (≈0.2−0.25), the energy upgrade of ReA up to ReA6 or ReA9 is

critical to have sufficiently high beam energies (Fig.1.4) for the measurements. Level

schemes and electromagnetic transition strengths will be determined for new candidate

nuclei and a possible presence or absence of octupole deformation can be studied. For

example, 229Pa is calculated to have an unusually small parity doublet splitting, which, if

confirmed, would make its observable atomic EDM several orders of magnitude larger than 199Hg. Nuclear structure studies which are sensitive to the relevant parameters are needed

to help both interpret EDM limits as well as identify the most promising candidates [Gaf13].

Fig. 6.1: Illustration of the reflection-asymmetric deformation observed in nuclei [Gaf13].

Coulomb excitation measurements have revealed the octupole collectivity of 224Ra,

pointing to a possible octupole-enhanced signal of an EDM in this nucleus.

27

7. Applications 7.1 Surrogate reactions Neutron-induced reactions on unstable nuclei are important for fundamental and applied

nuclear physics. Neutron capture reactions give rise to almost all of the elements heavier

than iron through both the slow s-process that occurs in AGB stars and the r-process that

occurs in explosive scenarios such as supernovae or neutron star mergers. The r-process

nucleosynthesis occurs in nuclei far from stability. The lack of knowledge of neutron

capture rates in nuclei near closed shells on the r-process path limits our ability to reliably

predict r-process abundance patterns following freeze out and in various r-process

astrophysical sites. Neutron capture also occurs in nuclear reactors on unstable species such

as fission products. And understanding neutron-induced reactions on unstable isotopes and

actinides is important for national security. Because pure neutron targets are not possible,

the only way to inform about neutron-induced reactions on short-lived isotopes and far

from stability is with a surrogate reaction involving reaccelerated rare-isotope beams. The

inverse kinematics (d,p) reaction is a promising surrogate for neutron-induced reactions

because of the positive Q-value (requiring lower beam energies) and that the recoiling

protons will preferentially be detected at backward angles in the laboratory, with no

competing direct reaction channels.

A specific example is the 130Sn(n,) reaction rate. An increase in this rate from standard

predictions can significantly impact r-process nucleosynthesis not only in the A≈130

region but also for heavier masses [Sur09]. The nuclide 130Sn is also an important 239Pu

fission fragment that has been used in modeling of nuclear devices. If indeed the 130Sn(n,)

cross section is significantly larger than predicted, fewer neutrons could be available for

other neutron-induced reactions in reactors and nuclear devices. The direct component of

the 130Sn(n,) has been measured [Koz12]. However, the statistical component, which

proceeds via a compound nucleus, could be orders of magnitude larger. A measurement of

the (d,p) surrogate reaction with 8-MeV/nucleon 130Sn beams could be used to populate

the 131Sn nucleus above the neutron separation energy. This reaction would require ReA9

beams of at least 8 MeV/nucleon. Rare-isotope beams would also provide the capability of

measuring surrogates for neutron-induced reactions on isomers. In the case of 130Sn, it is

likely that the 1.7-min 7− isomer of 130Sn will have significant ReA yields (the isomer ratio

was 10% for the 130Sn beam at HRIBF produced in proton-induced 238U fission [Jon16]).

Informing the 130mSn(n,) cross section could impact nuclear security applications since the

isomer could also be produced in neutron-induced fission.

Neutron-induced fission cross sections on short-lived actinides (ground states and isomers)

in the actinide region are also important for nuclear energy, forensics and national security.

Specific examples could be studies of (n,f) cross sections on 236mNp (t1/2 = 23 h) or 238Np

(t1/2 = 2.1 days) with a surrogate (d,pf) reaction. Actinide studies would require ReA12

beams to populate the final nuclei above the neutron-separation energy.

Other examples are neutron-induced reactions on 95Sr. This is a fission fragment used in

the modeling of nuclear devices and could also impact nuclear forensics since its daughter

is the relatively long-lived 95Zr (t1/2 = 64 days). The uncertainty in the 95Sr(n,) cross section

has been assessed to have significant implications on the production of 95Zr [LRPI15]. The

(d,p) surrogate reaction could be measured with 7-MeV/nucleon beams that would be

available at ReA6.

28

7.2 Nuclear structure from compound-nucleus reactions

Nuclear structure properties, such as the nuclear level density, -ray strength functions and

optical-model potentials, are required to understand compound-nucleus reactions far from

stability, as the direct study of these reactions is often challenging. In the compound

reaction mechanism, the projectile and target fuse to create an compound nucleus which

decays independently of its formation by emission of particles and rays. For medium-

mass and heavy nuclei, the compound mechanism almost entirely determines reaction cross

section at beam energies up to 6−9 MeV/nucleon. Compound-nucleus reactions also play

an important role in nucleosynthesis processes, in particular, for creating elements heavier

than iron in the cosmos. The theory of such reactions has been developed by Hauser and

Feshbach [Hau52] and suggests that the cross section of compound-nucleus reactions is

determined by nuclear structure input parameters such as transmission coefficients, nuclear

level densities, and -ray strength functions. For nuclei along the stability line, these

nuclear structure ingredients have been studied for many years and although they are still

the subject of continued studies to improve the accuracy of calculations, some regularities

are known. For nuclei off the stability line, however, experimental data are scarce and

theoretical extrapolations into this region are uncertain.

Experimentally, in a purely compound-nucleus reaction, nuclear structure ingredients can

be assessed from decay characteristics. With rare-isotope beams, neutron or proton-rich

compound states can be created by impinging rare-isotope beams on stable targets.

Generally, if beam intensities permit, nuclear level densities could be studied from double

differential cross sections of outgoing particles (neutrons, protons, α-particles) and the -

ray strength functions from particle- coincidences using the Oslo method [Sch00].

Another strong motivation to study neutron optical potentials for neutron-rich nuclei is

based on the experimental observation of Ref.[Tel71] that the neutron strength function,

which is related to transmission coefficients, decreases with increasing neutron number of

a particular element. This effect cannot be explained with existing parametrizations of the

optical model. Later, it was shown [Gor07] that if such decrease is extrapolated to neutron-

rich nuclei, it would result in a huge, up to several orders of magnitude, decrease in the rate

of the r-process nucleosynthesis, which is governed by neutron capture reactions. The

neutron strength function is determined by the imaginary isovector optical potential, which

is highly uncertain for unstable nuclei. Because there are no neutron targets to study the

interaction of rare-isotope beams with neutrons, the way to study optical potentials for

neutron-rich nuclei is to study decay channels of excited compound states. Specifically, the

ratio of proton/neutron cross sections in outgoing channels is determined by the ratio of

both level densities in corresponding channels and strength functions (optical potentials)

of outgoing particles. If the neutron strength decreases as one moves towards neutron-rich

nuclei, the cross section for outgoing protons and the ratio of proton/neutron cross sections

will increase according to the theory of Ref.[Hau52].

The above-mentioned studies to constrain nuclear structure quantities such as optical-

model parameters, nuclear level densities, and -ray strength functions from particle and

decay of compound neutron-rich or proton-rich nuclei are based on techniques established

with stable beams and can be applied to rare-isotope beams at ReA when optimal beam

energies of 3−9 MeV/nucleon become available.

29

7.3 Nuclear structure inputs for reaction rate calculations above A=200 Some of the major questions in both astrophysics and stockpile stewardship require

knowledge of neutron-induced reaction rates which are largely unknown, on nuclei which

are too short-lived to be studied directly. Theoretical calculations are an alternative, but

their reliability is ultimately limited by our incomplete knowledge of the nuclear physics

inputs such as optical potentials, nuclear level densities, and electromagnetic strength

functions. Recent work on 238U in forward kinematics has shown that -ray cascades

following reactions which populate levels near the neutron separation energy can constrain

the strength function and improve the theoretical calculations [Ull14]. The basic method is

shown in Fig.7.1, where the left panel shows measured capture -ray cascades (points) and

predictions using various input parameters.

Extending the basic approach from Ref.[Ull14] to reach short-lived nuclei can be

accomplished by using transfer reactions such as (d,p) in inverse kinematics at 5−10

MeV/nucleon to preferentially populate statistical (non-collective) states below the neutron

separation energy for the compound nucleus in question. Coincidences between the

recoiling charged particles and rays can then be used to constrain the strength function

and improve the nuclear reaction models. ReA12 beams resulting from FRIB would

provide unprecedented access to the necessary nuclei, particularly for the heavy mass

region near the actinides. Knowledge of the reaction cross sections in the mass 230−240

range would benefit applications of nuclear physics.

Fig.7.1: 238U capture -ray cascades (points) compared to theoretical models (dashed,

solid lines) (left) and measured cross section (black points) compared to calculations (right)

from Ref.[Ull14]. The model “GLO with M1” is clearly a better description of the cascades,

and ultimately using these parameters as inputs to a cross section calculation improves the

prediction significantly (right panel, blue line compared to green).

30

8. Equipment At the energy-upgraded ReA facility, state-of-the-art equipment and instrument including

GRETINA/GRETA, a solenoidal spectrometer, and ISLA will be installed for experiments.

In this section, the use of these equipment items at ReA is briefly described. In addition,

an extensive set of complementary and auxiliary detection systems is envisioned for ReA

experiments at the present NSCL facility and in the future at FRIB. A few examples of

such systems that are discussed in the preceding science sections are also given below.

8.1 GRETINA/GRETA The Gamma-Ray Energy Tracking In-beam Nuclear Array, GRETINA [Pas13], is

composed of more than 7 detector modules each of which has four 36-fold segmented Ge

crystals. As of early 2016, 9 modules have been available for experiments at NSCL

covering a solid angle range exceeding 1. When FRIB comes online, close to the full

4array GRETA (Gamma-Ray Energy Tracking Array) [GRE14] is anticipated for in-

beam -ray detection. Compared to fast-beam experiments with the strong Lorentz boost,

experiments with reaccelerated beams will remain in the low-energy domain where the

difference between the projectile and laboratory frames is less pronounced. Therefore,

experiments can fully exploit the advantage of the 4coverage, offering significant

advantages in -ray detection efficiencies for single and coincidence measurements. In

view of diverse reaction channels that are open at Coulomb-barrier energies, the installation

of recoil detection systems or a recoil separator is essential to achieve superior signal-to-

background ratios and to realize measurements of rare events.

Fig.8.1: GRETINA combined with CHICO2 (left) and a plunger device for lifetime

measurement (right).

A number of auxiliary detection systems will be used in combination with

GRETINA/GRETA at ReA. They include CHICO-type systems for the recoil particle

detection and a plunger system for lifetime measurements (Fig.8.1). CHICOx, a modified

version of CHICO2 [Wu16] is in the planning stage to alter the exterior design to fit inside

the cavity of GRETA. This unique instrument is useful not only for Coulomb excitation

studies but also for any experiments requiring measurements of two-body kinematics, such

as the quasi-elastic, deep-inelastic or fission reactions, for example. A plunger system

[Dew12,Iwa16] for lifetime measurements facilitates the recoil-distance Doppler-shift

method to measure excited-state lifetimes on the order of picoseconds. In experiments,

31

detectors at forward or backward angles are essential to fully exploit Doppler-shift effects

arising from recoil velocity changes due to the level lifetimes.

8.2 Solenoidal spectrometer The possibility to use pure rare-isotope beams at energies close to or above the Coulomb

barrier enables a number of reaction tools and studies that have been proven on stable

isotopes over many decades. The main difference when using rare-isotope beams is the

extensive use of inverse kinematics and the typically low available beam intensities. The

target nucleus is the (usually light) probe, and scattered particles have low energies. These

features require new types of experimental techniques where high luminosity is paramount.

In recent years, two innovative approaches to measure inverse-kinematics direct reactions

in the magnetic field of a solenoidal magnet were developed [Wuo07,Lig10,Suz12]. Such

studies with a solenoidal spectrometer are anticipated for ReA. One approach exploits

missing-mass spectroscopy at HELIOS [Wuo07,Lig10], utilizing the helical orbits of

recoiling particles that provide a linear relationship between the energy and the laboratory

angle. The other technique employs an active-target TPC [Suz12]. The advantage of these

new techniques is the achievement of very good excitation-energy resolution without

sacrificing luminosity, which is particularly important for direct-reaction studies with rare-

isotope beams at ReA.

A solenoidal spectrometer with charged-particle detection has demonstrated unique

features for nucleon transfer reactions in inverse kinematics. This technique, developed

several years ago [Lig10], is now well established. The method involves embedding the

charge-particle detection system into the uniform magnetic field produced by a large,

superconducting solenoid. The solenoid and magnetic field axes are along the beam

direction. The beam strikes a target on the solenoid axis and light charged particles such

as protons, deuterons, α particles, 3He or tritons follow helical orbits until they return to the

solenoid axis where they can be detected with position-sensitive silicon detectors. These

detectors measure the particle’s energy and the distance between the target and the point

where the particle returns to the solenoid axis. Although the relationship between kinetic

energies and laboratory-frame scattering angles is complex in inverse kinematics, in a

solenoidal geometry the relationship between kinetic energies and return distances is

simple and linear. Measurements of this type can provide excitation energies of high

resolution for the residual nucleus and enhance the sensitivity to weak transitions that might

otherwise be missed. The simple experimental scheme also makes the analysis of data

straightforward. Another advantage of this technique is that the recoiling heavy nuclei can

be detected in coincidence with the light-charged particles. This coincidence mechanism

not only reduces background, it can be used to isolate different decay modes.

The Active Target − Time Projection Chamber (AT−TPC) provides an alternative

approach for direct reaction studies at ReA [Suz12]. In this detector, a gas volume is used

both as a reaction target and detector medium. Because charged particles can be tracked

from the location where the reaction takes place (vertex), there is no loss of resolution or

dead layer of material depending on the target thickness as opposed to conventional inert

targets. In addition, the energy at which the reactions take place can be determined for each

event. This brings the possibility to efficiently measure excitation functions using the

slowing down of beam particles within the gas volume. The solid angle coverage

essentially approaching 4π is also a main asset of this technique that compensates for the

low intensities of rare-isotope beams as compared to stable beams.

32

8.3 ISLA A recoil separator has been identified as a key instrument to collect and identify recoiling

particles from low-energy reactions. In 2014, the community endorsed the Isochronous

Separator with Large Acceptance (ISLA) [ISLA15] which provides excellent M/Q

resolution (<1/1000) together with large acceptances in solid angle (64 msr) and

momentum (±10%). Based on the concept of the TOFI spectrometer [Wou87], ISLA is an

isochronous device where the time-of-flight from the target to the final focal plane is

independent of the energy and scattering angle after the reaction (Fig.8.2). The expected

M/Q resolution is on the order of 1/1000 allowing unambiguous identification of recoil

particles in a wide mass range. A high rejection power of the unreacted primary beam with

various charge states is also considered. A beam swinger is envisioned in front of the

spectrometer to cover grazing angles for two-body reactions at and above Coulomb-barrier

energies. Many of the experiments with major equipment such as GRETINA/GRETA and

ORRUBA (Fig.8.3) may be run in combination with ISLA.

A gas-filled mode for ISLA was also evaluated and found to be useful for a wide range of

kinematics in fusion-evaporation and multi-nucleon transfer reactions [ISLA15].

Fig. 8.2: A schematic view of the Isochronous Separator with Large Acceptance (ISLA) and

associated detector system described in the text boxes. The time-of-flight between the target

and the isochronous achromatic focal plane is measured using the charged-particle or -

ray array surrounding the target and the focal-plane timing detector, providing a direct

measure of the M/Q ratio of reaction products independent from their momentum vector.

33

8.4 Auxiliary detection systems A diverse set of complementary and auxiliary detection systems are anticipated for

experiments with reaccelerated beams at NSCL and FRIB. Examples of such devices

include ANASEN [Lin12] and ORRUBA [Pai07]. Experiments with these equipment

should be run in a stand-alone mode or in combination with other major equipment such

as GRETINA/GRETA and ISLA.

As an example of possible experiments at ReA, transfer reaction measurements can be

performed with the 2000-channel super ORRUBA array [Bar13], possibly coupled to a

single-pass gas-jet target, maximizing charged-particle resolution and minimizing target

effects (energy straggling, fusion-evaporation backgrounds). For measurements involving

higher level density, and surrogate measurements for statistical neutron capture, where

charged-particle detection alone is insufficient [Pai08], experiments including high-

resolution -ray measurements will be undertaken with the GODDESS system [Rat13].

The rays emitted in such reactions vastly improve the sensitivity and information yielded

in such experiments, and are critical to surrogate measurements for statistical neutron

capture [Hat10]. GODDESS (see Fig. 8.3 left) couples ~720 channels of highly-segmented

silicon detectors, providing ~30 keV energy resolution and better than 1 degree angular

resolution of 15 to 165 degrees for charged particles, with the high-efficiency germanium

detector array Gammasphere. For future programs, such as envisioned at the ReA facility

for r-process nuclei, a version of GODDESS using GRETINA is under development

(Fig.8.3 right), which will provide increased resolving power due to the position sensitivity

of GRETINA. This system, along with ReA beams, will provide unprecedented resolution

for transfer reaction studies with r-process beams.

Fig. 8.3: GODDESS (left) comprises the ORRUBA silicon-detector array, optimized for

transfer reactions, mounted inside Gammasphere. A coupling of ORRUBA to GRETINA

(right) is currently under development, which would be well suited to measurements at an

energy-upgraded ReA facility.

34

9. ReA energy upgrade 9.1 Conceptual layout of the ReA6-12 experimental area One possible conceptual layout of the energy-upgraded ReA facility is shown in Fig.9.1.

In addition to the existing experimental vault at NSCL, the design shown in Fig.9.1 is based

on the use of both areas north and south of the reacceleration cryomodules. When ReA12

is completed, two beam lines to the south will deliver reaccelerated beams to end stations

that may host a solenoidal spectrometer and ISLA, and additional beam lines are

envisioned for the general-purpose experimental area. The maximum magnetic rigidity of

the beam line is at least ~2.2 Tm to accept both neutron-rich and neutron-deficient beams

from ReA12 (Figs.1.4,1.5).

The ReA energy upgrade will be realized by extending the current ReA3 accelerator and

adding up to 3 new cryomodules. The energy specification for each step is shown in Fig.1.4.

Time or energy resolution of the reaccelerated beams can be controlled and improved if a

rebuncher is installed at the exit of the cryomodules before sending the beams to each

experimental area. Up to four independent experimental vaults are possible which will be

separately shielded to avoid conflicts between ongoing runs and experiment preparations.

Fig.9.1: Possible layout plan for ReA6-12 in the existing experimental hall of NSCL.

35

9.2 Cost estimates for the ReA energy upgrade The cost of the ReA energy upgrade as well as the associated infrastructure and diagnostic

systems were estimated based on the layout plan described in the previous section. These

estimates are approximate and detailed cost estimates have not been developed. These costs

include procurement, manufacturing, installation and labor costs. Contingencies of 25%

were added for magnets and associated components. These are summarized in Table 9.1

below. The cryogenic loads are uncertain and additional cryogenic capacity may be needed

in addition to the listed items in Table 9.1. A 40T crane is available in the ReA experimental

area, but new vault shielding (walls and roof beams) needs to be implemented for each

experimental area as described in the layout plan (Fig. 9.1).

Table 9.1: Summary of the cost estimates for the ReA energy upgrade up to ReA12

Item Cost Note

Beam line

magnets and

components

$6.1M Including beam line

dipole and quadrupole

magnets and associated

infrastructure

Cyromodules $17.0M Including rebunchers

Vault

Construction

$1.9M Shielding and roof

beams

Total $25.0M

9.3 Staging options Staging options are considered where the ReA energy is upgraded in a maximum of three

steps depending on the available funding for cryomodules and associated infrastructure.

Starting with the current ReA3 beam line, each additional cryomodule increases the

available beam energy by at least 3 MeV/nucleon (Fig. 1.4), defining the energy profiles

for ReA6, ReA9 and ReA12, respectively. Each stage utilizes the same experimental area

as described in the layout shown in Fig. 9.2, but associated beam lines and infrastructure

are to be installed in steps. Separate budget scenarios would amount to $8.5 M, $6.9 M,

and $9.6 M for the ReA6, ReA9 and ReA12 upgrades, respectively. The ReA6 upgrade

project includes the construction of one experimental area including shielding. One beam

line can be dedicated to large-scale equipment such as GRETINA/GRETA and a solenoidal

spectrometer. In addition, in this scenario one general-purpose beam line and associated

infrastructure will be prepared. The ReA9 upgrade includes the addition of one cryomodule

as well as an additional beam line to a separate vault where ISLA can be accommodated.

Finally, the ReA12 upgrade completes the set of three cryomodules and installs the beam

lines which could host large-scale equipment such as GRETINA/GRETA and a solenoidal

spectrometer in an independent dedicated area. Gamma-ray and particle detection systems

can also be used in combination with the ISLA recoil separator. On the north side, there

will be additional general-purpose end stations for user-developed equipment with strong

complementary capabilities and a possible full extension within the existing high bay area

is illustrated in Fig.9.1. A pre-conceptual layout for the intermediate upgrades ReA6 and

ReA9 is shown in Fig.9.2.

36

(a)

(b)

Fig.9.2: Staging options of the ReA energy upgrade. Possible layout plans are shown for

(a) ReA6 (top) and (b) ReA9 (bottom), respectively.

37

ReA3 upgrade workshop

ReA3 upgrade workshop held in August, 2015 on campus of Michigan State University

A one-day workshop to discuss science opportunities with the energy upgrade to NSCL’s

ReA3 accelerator was held on August 20, 2015, on the campus of Michigan State

University, preceding the 2015 Low Energy Community Meeting (LECM). The event was

very well attended with more than 70 registered participants. A high energy upgrade to

ReA6 and eventually to ReA12 in the future is one of the flagship projects at NSCL and in

the future at FRIB. A timely construction of ReA12 has been strongly endorsed in the 2014

Low-Energy Nuclear Physics DNP town meeting and the long-standing interest of the

community was reaffirmed by the large number of participants in the workshop and again

articulated in the close-out of the 2015 LECM. Following the opening and overview talks

on the ReA facility, 14 speakers presented science opportunities that will open up with

energy upgrades to ReA6−12.

38

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41

Contributors

Contributors to the ReA energy upgrade whitepaper and registered participants for the workshop

Sunghoon Ahn Steven Grimes William Peters

Tan Ahn Terry Grimm Marina Petri

Matt Amthor Carl Gross Thomas Redpath

Daniel Bazin Kalee Hammerton Grigory Rogachev

Saul Beceiro Novo Christine Hampton Krzysztof Rykaczewski

Peter Bender Jessica Harker Konrad Schmidt

Georg Berg Calem Hoffman Jasmine Sethi

Jill Berryman Sylvie Hudan Bradley Sherrill

Carl Brune Hiro Iwasaki Anna Simon

Alexander Burger Benjamin Kay Michael Smith

Chris Campbell Nobuyuki Kobayashi Artemis Spyrou

Michael Carpenter Zach Kohley Chris Sullivan

Kelly Chipps Filip Kondev Chandana Sumithrarachchi

Jolie Cizewski Amel Korichi Michael Syphers

Kortney Cooper Elaine Kwan Rashi Talwar

Marco Cortesi Hye Young Lee Wanpeng Tan

Manoel Couder Charles Loelius Oleg Tarasov

Aaron Couture Walter Loveland Paul Thompson

Heather Crawford William Lynch Betty Tsang

Mario Cromaz Augusto Macchiavelli Robert Varner

Jiban Jyoti Das Paul Mantica Antonio Villari

Paul De Young Scott Marley Alexander Voinov

Catherine Deibel Alan McIntosh Aditya Wakhle

Brandon Elman Wolfgang Mittig William Walters

Paul Fallon David Morrissey Kenneth Whitmore

Charles Folden Shea Mosby Ingo Wiedenhoever

Alexandra Gade Farheen Naqvi Ching-Yen Wu

Jacklyn Gates Witold Nazarewicz Alan Wuosmaa

Tom Ginter Wei Jia Ong Sherry Yennello

Thomas Glasmacher Steven Pain Remco Zegers

Kenneth Gregorich Georgios Perdikakis Vladimir Zelevinsky

Uwe Greife Jorge Pereira Qiang Zhao